1. Field of the Invention
The present invention is directed to an X-ray examination apparatus of the type having an x-ray tube having an anode dish with optionally activatable focal spots that are arranged such that all are intersected by a common straight line, and having a displaceable primary radiation diaphragm.
2. Description of the Prior Art and Related Subject Matter
German OS 3 136 806, corresponding to U.S. Pat. No. 4,464,778, discloses an x-ray examination apparatus of this type. The anode dish in the x-ray tube, which is rotatable around its own axis, is provided with two different focal spot paths inclined relative to the dish plane. The x-ray tube also has two cathodes, each cathode being directed onto a respective focal spot path. A focal spot is respectively generated on each focal spot path by the cathodes, and the focal spot of the one path is, for example, larger than that of the other path. As a result, the operator can activate that cathode and thus that focal spot, which the operator considers most suitable in an examination, for example in a mammography examination. Since the focal spots are present on the anode dish with a specific spacing from one another, the primary radiation diaphragm must also be displaced when a switch is undertaken from the one to the other focal spot so that the x-ray cone remains directed onto the subject table, i.e., so that it exactly covers the sheet of x-ray film that is inserted into or under the subject table. In the aforementioned German published application, the primary radiation diaphragm or the primary radiation diaphragm housing, can only be laterally displaced up to two limit positions, whereby the primary radiation diaphragm is set with reference to one focal spot in the one limit position and is set with reference to the other focal spot in the other limit position. It has not been taken into consideration in this x-ray examination apparatus that these focal spots, in addition to being offset from one another in a lateral spacing at the anode dish, are also offset spaced from one another in height, i.e. with different spacings from the primary radiation diaphragm or from the subject table. Switching to a focal spot that lies closest to the primary radiation diaphragm thus can lead to an undesirable spread of the x-ray field outside the exposure region.
The Siemens brochure "Mammomat" describes a mammography apparatus having an x-ray tube that is disclosed in German OS 3 136 806. The apparatus is also provided with a primary radiation diaphragm housing that is pivotable into two limit positions with articulated arms, so that the primary radiation diaphragm is matched to the current focus point in each limit position. As a result of the primary radiation diaphragm housing describing an arcuate motion given a displacement from one to the other limit position, it is displaced laterally as well as in height. In this way, the primary radiation diaphragm is also matched in height for the focal spot that lies closest to the subject table.
Co-Pending U.S. patent application Ser. No. 08/245,249 ("X-ray Tube with Multiple Differently Sized Focal Spots, Knott)" filed Mar. 3, 1994, discloses an x-ray tube with an anode dish at which four focal spots can be generated by the same number of cathodes. The focal spot paths of the anode dish lie on a common plane that is inclined relative to the dish plane, so that all four focal spot that can be generated are intersected by a common, imaginary straight line that proceeds along the inclined plane. A primary radiation diaphragm that is employed in conjunction with the mammography apparatus described in the Siemens brochure "Mammomat" cannot be used in combination with the anode dish disclosed in this unpublished German patent application since this primary radiation diaphragm can only be brought into two limit positions that are respectively matched to one focal spot. Due to the arcuate motion that the primary radiation diaphragm describes given a displacement from one to the other limit position, there is no position between the two limit positions wherein the diaphragm can be matched to another focal spot capable of being produced at the anode dish, since all of the focal spots are intersected by a common straight line.